This invention relates to a method of treating the surface of a shaped body formed of a polypropylene resin.
Excellent mechanical and physical properties of polypropylene resins allow the use thereof for interior or exterior decorative applications in, for example, automobile parts such as instrument panels, trims, pillars and bumpers and domestic electric appliances such as vacuum cleaners.
However, because of their non-polar, highly crystalline nature, the polypropylene resins pose a problem of difficulty in coating, printing, bonding and the like surface processing. To cope with this problem, there have been proposed the following pretreating methods: primer coating, sand blast treatment, chromic acid treatment, flame treatment, corona discharge treatment, plasma treatment, functional groups-introducing treatment and photo-grafting treatment. These conventional methods, however, are not entirely satisfactory for the reasons as set forth below.
The primer coating method includes applying a coating of a primer composition, such as a toluene solution of a chlorinated polypropylene, over the surface of a shaped polypropylene article. Primer compositions are generally expensive. Further, the primer coating is effective to only specific types of coating materials to be used in succeeding main treatment steps.
The sand blast method is one in which granules of a grinding material are caused to impinge on a surface of a shaped body to be treated for roughening same. The granular grinding material causes fouling of the working environment and the product. The surface of the treated product is, therefore, required to be washed with water. Further, there are caused problems that the treatment makes the surface to be treated opaque and that the grinding material once cut into the surface to be treated cannot be easily removed.
The treatment with chromic acid mixture includes heating the chromic acid mixture, which, for instance, contains 75 parts of potassium bichromate, 120 parts of water and 1500 parts of concentrated sulfuric acid, to about 100 .degree. C., and immersing a shaped body to be treated in the heated mixture for 10 about 5 minutes. This method requires high cost for the treatment of the waste chromic acid mixture.
The flame treatment includes exposing the surface of a shaped body to be treated to an oxidizing flame with a temperature of 1000.degree.-2500 .degree. C. produced by combustion of a fuel gas at an excess air ratio. The treated surface is liable to be distorted or melted by the heat.
The corona discharge treatment includes passing a film or a film-like body to be treated through a space between two opposing electrodes while applying a high electric voltage therebetween. This method is not applicable only to flat bodies.
The plasma treatment includes exposing the surface of a plastic body to be treated to a low temperature plasma, so that the surface undergoes a chemical change by the action of dissociated oxygen and ultraviolet rays. Plasma of oxygen or air is used. This method is disadvantageous because of the necessity of an expensive apparatus.
The functional group-introducing method includes irradiating UV rays on the surface of a shaped body to be treated in an atmosphere of chlorine gas, and then treating the irradiated surface with an alkali. This method poses a problem of handling very dangerous chlorine gas.
The photo-grafting method includes admixing benzophenone with polypropylene powder, forming the admixture into a film, and photo-treating the surface of the film in the presence of acrylamide in an oxygen-free atmosphere for photochemically graft-copolymerizing the acrylamide with the polypropylene. This method requires complicated steps and, therefore, is economically disadvantageous.